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Zusammenfassung
Solvation dynamics of 4-aminophthalimide (4AP) in methanol is measured by broadband upconversion of the fluorescence band. The peak emission frequency n(t) is detd. from 100 fs onward with 85 fs time resoln. Polar solvation based on simple continuum theory, including solute polarizability, describes the temporal shape of n(t) quant. Extrapolation n(t->0) points to an initial emission frequency ...
Zusammenfassung
Solvation dynamics of 4-aminophthalimide (4AP) in methanol is measured by broadband upconversion of the fluorescence band. The peak emission frequency n(t) is detd. from 100 fs onward with 85 fs time resoln. Polar solvation based on simple continuum theory, including solute polarizability, describes the temporal shape of n(t) quant. Extrapolation n(t->0) points to an initial emission frequency which agrees with the result from stationary spectroscopy in a nonpolar solvent. The extent (4300 cm-1) of the dynamic Stokes shift is largely due (50%) to H-bonding, however. The observations imply that H-bonds with 4AP adiabatically follow the dielec. relaxation of the methanol network. The stimulated emission band is also used to measure solvation dynamics. The evolving band is monitored by transient absorption spectroscopy of supercontinuum probe pulses. But the excited-state absorption spectrum, its relative amplitude, and its evolution are needed to ext. n(t) from such measurements. These key data are obtained by comparison with the upconversion results. Thus calibrated photometrically, 4AP transient absorption can be used to monitor solvation dynamics in any solvent. The excited-state absorption spectrum is assigned with the help of time-dependent d.-functional calcns. Fluorescence excitation and double-resonance spectroscopy of isolated 4AP, cooled in a supersonic jet, is used to det. optically active modes. An intramol. reorganization energy is inferred which is consistent with the value in 2-methylbutane (2025 cm-1). The crystal structure is also provided.